Keep the clutter to a minimum with financial blog Wise Bread's principle of "like in, like out":

If a new item comes into the house, like a sweater or souvenir coffee mug, make sure something similar goes out either to the recycling center, Goodwill, to a friend or if necessary to the dump. This will keep you from re-establishing clutter after you have put forth the effort to downsize.

You could also try the penicillin method
or follow the 15 minute rule
to get rid of your clutter. What's your best declutter strategy? Please share in the comments.